Accurate chromosome segregation is required for propagation of all cells. Errors in this process are implicated in oncogenesis, birth defects and cell death. Crucial to proper partitioning of the chromosomes during cell division is the establishment, arrangement and then breakdown of a bipolar spindle. The aim of this grant is to further understand the structure, assembly and dynamics of the proteins that create and control the organization of the spindle in the model organism Saccharomyces cerevisiae. The microtubule organizing center in yeast is the spindle pole body (SPB). The SPB is a dynamic structure that undergoes remodeling in a cell-cycle specific manner. We have identified proteins involved in the remodeling process and will characterize them. We have provided a detailed architectural description of the SPB and will continue our structural analysis. The SPB nucleates and organizes microtubules. We will perform a set of experiments designed to distinguish between two current models for nucleation. Finally, we have found that kinetochores can biorient without being attached to the plus-end of microtubules. New insights into biorientation will be obtained from determining the role of kinetochores, motors and other spindle proteins in the formation of the bipolar spindle when plus-end attachment is not involved. Because many proteins and spindle features are conserved from yeast to human, our studies will inform models of how eukaryotic cells correctly distribute their genetic material.